1. Field of the Invention
The present invention relates to systems and methods for delivering signals, such as video signals, over a signal carrier, and systems and methods for compensating for the losses sustained by those signals in their delivery.
2. Description of the Related Art
When delivering and/or transmitting a signal over a signal carrier and over some distances, whether the signal carrier be a signal-carrying wire or a channel in wireless communication, the delivered/transmitted signal inevitably suffers loss, or attenuation, during delivery/transmission through the signal carrier medium.
One method of compensating the attenuation is to measure the received signal, and feed the signal or other information back to the source of the signal for compensation. However, such a solution is disadvantageous where there may be a plurality of receiving points along the delivery/transmission path. Because the loss at each of the receiving points may be different, the signal source cannot be amplified to compensated properly for all the receiving points.
The attenuation issue is exemplified by the delivery of the video signal, such as the standard definition (SD) video signals in National Television System Committee (NTSC), Phase Alternating Line (PAL), and SECAM standards. The issue is exacerbated with emerging high definition (HD) video signals, which operate at over 30 MHz, and sustain significantly more attenuation in the delivery thereof. Viewers are affected by the signal loss, because the quality of the pictures suffers if the losses are not compensated.
The video signals may be carried on a physical medium, such as a cable. One type of video signal cable is the twisted-pair cable, such as the Category 5 cable (CAT-5). The CAT-5 cable includes four individual twisted-pair cables in a single cable jacket. Another type of cable is the coaxial cable. The twisted-pair cable has a pricing advantage over the coaxial cables, such as RG-6 cables. For delivering component video having Y, Y-r, and Y-b signals, three coaxial cables are typically needed. Whereas, only one four-twisted-pair cable is needed to carry the same signals.
Moreover, a video delivery system using coaxial cables typically uses single-ended drivers and receivers. In contrast, a system using twisted-pair cables typically uses differential drivers and receivers, and therefore offers the advantage of reduced ground loop noise. In particular, common mode noise from local interference may be canceled out at the receiving ends in a video delivery system with twisted-pair cables.
A twisted-pair cable having multiple twisted-pair wire lines, such as the CAT-5 cable, offers an additional advantage of having a fourth twisted-pair that can be used to carry a companion audio feed, such as that of Sony/Philips Digital Interface format (SPDIF) digital audio output. Most DVD players and HD set-top boxes have an SPDIF digital audio output in addition to the component video outputs.
Moreover, many buildings have been pre-wired with CAT-5 cables, and utilization of those cables for video application can minimize the installation cost and effort.
Accordingly, it has become increasingly desirable to use twisted-pair cables for video delivery. Twisted-pair cables such as CAT-5 cables have until now been associated with LAN network interconnectivity, and using this type of cable for video signal requires the design of appropriate cable drivers and receivers.
However, twisted-pair cables suffer from the aforementioned signal attenuation issue far more than the coaxial cables, even for the highest grade CAT twisted-pair cables. For example, Belden 1872A “MediaTwist” cable is marked “tested to 350 MHz.” However, the signal attenuation for that cable is rated 39.8 dB for a 100-meter length at 350 MHz. For 60 MHz signals, the attenuation is still rated at 15 dB.
Another prior solution to the attenuation problem of carrying video signals over twisted-pair cables include using a receiving amplifier with adjustable frequency gain. The system installer would manually set several switches that control the gain of the receiving amplifier, for specific lengths of twisted-pair cables. The switch settings are dependent on the length and grade of the particular cable, due to variations of signal attenuation that exist in each cable. An installer would might measure the signals using specialized signal generation and measurement equipment to appropriately set the switches for each installation. The switches generally provide a discrete form of compensation that is invariably less than optimal. Furthermore, as signal transmission conditions change, such as may and are likely occur with changes in temperature, such manual customized compensation often loses much or all of its intended effect. Moreover, manually setting the gain level of receiving amplifiers is subject to human error and cannot be assured for each and every installation.
Also, low frequency signal loss, which is largely caused by the I-R drop of the cable, has not been adequately resolved by prior solutions. Video displays are expected to correct the low frequency signal loss by measuring the synch-tip amplitude and resealing the video to compensate for the loss.
Prior solutions thus require a combination of substantial time, skill, and/or equipment for installing each cable. Accordingly, a system that can compensate for signal loss in a signal carrier at multiple receiving locations is needed. Moreover, a system that can automatically compensate for signal loss independent of the physical characteristics of the carrier media is also needed.